London: Covert nuclear tests may soon become a thing of the past, thanks to pure samples of a telltale gas that could help nuke sniffers.

A global network of monitoring stations continually samples the air for signs of underground nuclear tests.

One thing these stations look for is the radioactive gas xenon-133.

Nuclear explosions produce an excited form called xenon-133m, in which the atomic nucleus is boosted to a higher-energy state.

However, it is not known exactly how sensitive detectors are to this form because until now there has been no way to make pure samples of xenon-133m with which to test them.

Creating pure samples of the gas produced by nuclear tests will make monitoring more reliable.

Now Kari Peräjärvi of the Radiation and Nuclear Safety Authority in Helsinki, Finland, and team have solved the problem, reports the New Scientist.

The researchers placed a cloud of xenon-133 atoms inside a magnetic trap and then jolted it with oscillating electric and magnetic fields.

This pushed out the unexcited form, leaving only the excited form behind.

The work could make nuclear monitoring with air samples more reliable.

However, James Acton of the Carnegie Endowment for International Peace in Washington DC believes the gas may stay trapped below ground if there are no cracks for it to seep through, making on-site visits - not currently required by international law - a better approach.